If the "woman" in the OP has had the reassignment surgery, then no problem.
If not, and the person is still biologically a male, then stay out of the women's rest rooms and locker rooms.
Simple really.
No amount of surgical mutilation will change the biology of any person. But I agree that if it has had the gender-appropriate equipment altered in such a way as to diminish its potential threat to women and girls, and it is discreet, it may go un-noticed when it enters and uses the bathroom of its choice.